As is well known, MR imaging uses an RF receive coil to receive the signals emitted by the subject under test in response to excitation of a selected volume of the subject which is generated by a RF transmit coil, such as the built in body coil. Thus the Gradient coils generate controlled variations in the main magnetic field (B0) magnetic field to produce selected spatial excitation volume and the signal emitted by that selected volume is picked up by the receive coil arrangement and transmitted to a signal processing system.
The receive coil arrangement can comprises a single coil loop or element or it can include a series of loops arranged in a pattern around the part of the subject to be imaged.
MR systems provide a built in body coil in the magnet construction and this can operate as both the transmit coil and the receive coil.
However in some cases the body coil does not provide an image of sufficient quality to meet the requirements and hence local coils must be used. These are typically volume coils which are configured to at least partially or completely surround the region of interest of the subject so as to receive the MR signal and include a plurality of connected conductors.
Some current volume coils consist of coil loops, phased array, birdcage, TEM, all of which could be single frequency or dual frequency coils. These require matching networks, preamplifiers, decoupling networks, cables and connectors.
There are a number of challenges with the current standard volume coil designs:
a) The number of channels is limited to the number of receivers in the system.
b) A large diameter cable bundle, such as an eighteen channel phased array coil which require 18 channel cables, containing 18 coaxial cables and at least 25 control wires, would be much too large to enable construction of the conventional cable trap in the cable.
c) It is difficult to build because the electrical components, such as the circuit board baluns and preamps, are complicated and time consuming to assemble by a skilled and experienced technician. These components require significant effort during design and construction to produce high quality images and to reduce the crosstalk between components.
d) The required mechanical components, such as the long cables, cable traps, and connector interface also increase the overall size and weight of the coil.
e) The large size and weight of the coils increases complexity of workflow for customer and complexity of the workflow design.
f) Long cables are heavy and cumbersome to position.
g) There are patient positioning and surgical access issues due to the inflexibility of the current design and the ever-changing surgical requirements and surgeon's preferences.
h) Coil cables have the possibility of patient burns resulting from skin-to-cable contact, resulting in increased space between cables, magnet bore and patient. This provides less in less patient space for nursing staff to properly position the patient before scanning.
i) In an inter-operative suite, there are safety issues related to OR staff forgetting to unplug the coils and increased OR workflow due to the additional patient safety checkpoint.
Normally, each individual loop or loops of the MRI receive coil arrangement are connected to a single receiver of the signal processing system via preamplifier and other components with a cable.
Such receive coil arrangements can therefore use the so called “built in body coil” carried on the magnet as receive coil which is connected by cable to the signal processing system. In this case the so called “built in body coil” is also used as transmit coil
Such receive coil arrangements can therefore comprise a single loop which is connected by a single wire to a single channel of the signal processing system. In this case the system can use the so called “built in body coil” carried on the magnet as transmit coil. This signal loop receive coil then supplies the received signal collected around the subject, typically a lying patient, and communicates it to the single channel for processing using conventional systems well known to persons in this art.
Such receive coil arrangements can therefore comprise a multiple loop arrangement including a so-called “phased array” of loops each of which is connected by a respective wire to a separate one of a plurality of channels of the signal processing system.
In this case the system typically uses a portable coil assembly arranged to wrap around the body part of the patient but each loop must have its own set of processing components and its own wire connecting the signal to the separate channel for processing.
However in recent developments not yet widely adopted, the “built in body coil” carried on the magnet as the receive coil arrangement is separated into individual loop components for supplying a separate signal to the separate channels.
It is well known that there are parallel imaging techniques to reduce the time necessary to obtain a complete scan of the part of the patient by using the signals from the separate channels to carry out various calculations and extrapolations, thus avoiding the necessity to obtain image results at each location in the image space or in K-space. Some of these parallel imaging techniques are known as SMASH and SENSE and GRAPPA.
To obtain better images, the preamplifiers are located as close to the coil elements as possible. Although the size of MR preamplifier is greatly reduced recently, it still takes much space of overall array coil. In addition the area of coil enclosure at preamplifier must be rigid.
The coil cable, as is well known, consists of multi-coaxial cable and signal control wires and outer shield. Common mode current or shield current will be generated on outer surface of the shield during transmit phase by the high RF field generated by the transmit coil. To prevent the patient from being overheated dangerously by shield current, cable traps are required for the coil cable assembly. Longer cable with more cable traps is required for the clinic applications, such as intra-operative MR imaging on a moving magnetic system.